Coriolis type mass flow meters operate on the principle that fluent material passing through a conduit tubing, when exposed to a deflection or oscillation transverse to the direction of flow, will react with a measurable force (the Coriolis force) on the walls of the tubing. The Coriolis reaction is generated by the material moving in an instantaneously changing curvilinear path, and acts with a force directly proportional to the momentum of the material in the tubing. The Coriolis reaction tends to apply a force in the opposite direction of the motion of the deflection of the flow tube on the inlet side thereof and with an equal and oppositely directed force on the outlet side thereof. The measurement of this reaction is used to determine the mass flow rate passing through the flow tube.
The flow tube portion of the Coriolis mass flow meter of the present invention is preferably formed in accordance with commonly assigned, co-pending application Ser. No. 912,893, filed Sep. 26, 1986. A Coriolis flow meter of this type includes a stabilized flow tube having a centralized center of gravity position substantially proximal the center of gravity of its mounting. This centralized mounting provides a stable oscillating structure which is less susceptible to sensor signal contamination, due to external noise or vibrational influences unrelated to the applied oscillation. A stable flow tube form permits more flexible tubing to be utilized, increasing sensitivity without increasing the detrimental effects on the Coriolis reaction measurement due to external mechanical noise and the like from the flow meter environment. Additionally, the present invention contemplates an oscillation applied to the flow tube at a frequency that is higher than the natural fundamental resonance of the flow tube. This applied oscillation produces a vibrational wave pattern along the flow tube length.
Increasing flexibility of the flow tube within a Coriolis mass flow meter increases the sensitivity of the meter to the Coriolis reaction, since the tubing will more readily react in response to the Coriolis force of the fluid. However, the detrimental effects of outside mechanical and vibrational noise on the Coriolis reaction measurement, is also typically increased by a more flexible flow tube. Further, the lateral stability (i.e. transverse to the direction of the applied oscillation) of the flow tube may also be detrimentally effected by increasing flexibility, introducing errors in the sensor signals as well as effecting the frequency of oscillation applied to the flow tube by the driver.
U.S. Pat. No. 4,711,132, issued Dec. 8, 1987, in FIGS. 11 and 12 shows stiffening portions on the flow tube which are intended to resist what is termed "wobble" and "roll" created by lengthening the projection of the flow tube from its mounting to increase flexibility. FIG. 11 shows stiffening plates which are attached to the sides of the projected flow tube to resist curvilinear bending, called "wobble", of these sides during application of the driver oscillation. FIG. 12 shows struts fixedly attached at both ends to two positions on the flow tube on the same side of the applied oscillation. These struts are intended to prevent displacement of the flow tube laterally, called "roll", with respect to the plane thereof. These struts and stiffening plates also limit the ability of the flow tube to flex in response to the Coriolis reaction of the fluid. These structures, therefore, are considered inapplicable to flow meters for measuring low flow rates, since sensitivity is an important feature within this flow range.